The present invention relates to a display apparatus including a CRT.
FIG. 4 shows a structure of a conventional CRT display apparatus. In this figure, there is shown a CRT 14, a cathode 2, a G1 electrode 3, a G2 electrode 4, a G3 electrode 6, an anode 7, a video circuit 9, a flyback transformer (FBT) 15, and a variable resistor 16. The G1, G2, and G3 electrodes are cylindrical-shaped electrodes disposed within the electron gun to draw electrons from the cathode and prefocus them. Another focusing electrode and the like disposed after the G3 electrode are omitted from the drawing to simplify explanation.
The operation of the apparatus of FIG. 4 will be explained below. A video signal is amplified by the video circuit 9, and is supplied to the cathode 2. A high voltage of about 25 KV, which the FBT 15 produces by stepping up and rectifying horizontal flyback pulses generated in a not-illustrated horizontal-deflection-signal output circuit, is applied to the anode 7. The G2 electrode 4 is applied with a voltage of 700V to 1000V which the variable resistor 16 produces by dividing the high voltage. The current flowing through the G2 electrode 4 is very small, and therefore the variable resistor 16 has resistance as much as 100 Mxcexa9. By adjusting the voltage applied to the G2 electrode 4, a coarse adjustment to a threshold point at which electrons start to flow towards a screen and the screen starts to illuminate, which is called a screen adjustment, can be carried out.
Generally, a CRT display apparatus is provided with a facility of adjusting brightness and a facility of adjusting contrast. The brightness adjusting facility enables a user to tune a black level of a picture and the threshold point at which the screen starts to illuminate to his liking. Generally, this brightness adjusting facility is obtained by changing a black bias voltage of a video signal supplied to the cathode. The contrast adjusting facility enables a user to tune the ratio of a brightness of the darkest part to that of the brightest part in the picture to his liking. Generally, this contrast adjusting facility is obtained by changing the amplitude of a video signal supplied to the cathode.
On the other hand, the demand for improving intensity and resolution of a CRT display apparatuses is growing in recent years. Japanese Unexamined Patent Publication No. 11-224618 discloses a high intensity/resolution CRT (referred to as a xe2x80x9cHi-Gm tubexe2x80x9d hereinafter) that addresses such a demand. This Hi-Gm tube features a novel electron gun that has, in addition to the G1, G2 and G3 electrodes, a modulating electrode called xe2x80x9cGm electrodexe2x80x9d disposed between the G2 electrode and the G3 electrode.
FIG. 5 shows a structure of such an electron gun used for the Hi-Gm tube. In this figure, 17 denotes a G1 electrode, 18 denotes a G2 electrode, 19 denotes a G3 electrode, 20 denotes a cathode, 21 denotes an electron-emitting substance formed on the surface of the cathode, and 22 denotes a Gm electrode. This electron gun has, for the part following the G3 electrode where another focusing electrode and the like are disposed, the same structure as the conventional electron gun.
FIG. 6 is a graph showing potential distribution in the vicinity of the cathode within the electron gun of the Hi-Gm tube.
In this graph, the horizontal axis represents the distance (mm) from the cathode surface, the vertical axis represents the potential (V), and the curve 23 shows the potential distribution symmetrical with the axis of revolution in the vicinity of the cathode. Furthermore, the arrow 24 shows a range within which the Gm electrode 22 exists, which is about 0.5 mm from the cathode surface. The graph of FIG. 6 holds while the G1 electrode is applied with 0V, the G2 electrode is applied with 500V, the G3 electrode is applied with 5.5 KV, the Gm electrode is applied with 80V, and the anode is applied with the high voltage of 25 KV for example.
The potential of the Gm electrode 22 is set to about 80 VDC, so there is a position 25 within the range 24 at which the potential curve 23 is minimum. If the potential of the cathode 20 shown by the broken line is lower than the potential at this position 25, electrons pass through the position 25 and flow towards the screen. If not, electrons do not flow towards the screen since they cannot pass through the position 25.
As seen from this graph, between the cathode 20 and the position 25, electrons always exist abundantly, and the potential slope after the position 25 is of the order of 106 (V/m). Compared with the potential slope between the cathode and the G1 electrode, it is greater by an order of magnitude. Accordingly, after electrons pass through the Gm electrode 22, most of them can move towards the screen without being affected by spatial charges, so the intensity of the electron beam flowing to the screen is determined by the quantity of the electrons that pass through the position 25 at which the spatial potential is minimum.
For this reason, variation of the intensity of the electron beam when the cathode potential is varied by a certain value in the Hi-Gm tube is about twice as much as that in the conventional CRT. That is, the variation of the cathode potential required to vary the intensity of the electron beam by a certain value in the Hi-Gm tube is less than half the variation required in the conventional CRT. In other words, with the Hi-Gm tube, the variation of the intensity of the electron beam can be doubled for the same variation of the cathode potential. Consequently, with the Hi-Gm tube, it is possible to easily adapt to video signals of high frequency, and therefore to provide a display apparatus of high intensity and high resolution.
Although it has been described that, in a conventional CRT display apparatus, brightness of a picture is adjusted by changing the value of a black bias voltage supplied to the cathode, it is necessary in reality, to change each of three bias voltages of the three channels of R, G, and B. Furthermore, in the case of adjusting contrast of a picture by changing the amplitude of a video signal supplied to the cathode, to obtain high contrast, expensive amplifiers having a high gain and an expensive power supply outputting a high voltage are required.
An object of the present invention is to provide a display apparatus in which brightness is adjusted by a simple circuit utilizing the above-described characteristics of the Hi-Gm tube. Another object of the present invention is to provide a display apparatus capable of displaying a picture in high contrast at a low cost.
The above-described object is achieved by a CRT display apparatus comprising:
a CRT including an electron gun,
the electron gun having a cathode, a G1 electrode, a G2 electrode and a G3 electrode disposed in that order to draw electrons from the cathode, the electron gun further having a modulating Gm electrode disposed between the G2 and G3 electrodes; and
a controller for controlling a value of a voltage applied to the Gm electrode to adjust brightness of a picture on a screen of the CRT.
The controller may be a voltage source which produces a voltage having a value corresponding to a value of a brightness adjustment signal input to the voltage source, and applies the produced voltage to the Gm electrode.
The above-described another object is achieved by a CRT display apparatus comprising:
a CRT including an electron gun,
the electron gun having a cathode, a G1 electrode, a G2 electrode and a G3 electrode disposed in that order to draw electrons from the cathode, the electron gun further having a modulating Gm electrode disposed between the G2 and G3 electrodes; and
a controller for controlling a value of a voltage applied to the G2 electrode to adjust contrast of a picture on a screen of the CRT.
The controller may be a voltage source which produces a voltage having a value corresponding to a value of a contrast adjustment signal input to the voltage source, and applies the produced voltage to the G2 electrode.